Persistent ionization in air (PIA) plasmas are plasmas that are formed at atmospheric pressures and that persist for a finite time after termination of the power source. Large volume PIA plasmas have generated research interest because they are useful for simulating a phenomenon known as ball lightning, which is commonly observed in thunderstorms. In ball lightning, air and other gases are observed under certain conditions to have high levels of ionization for periods that are very long compared to the recombination times of the electrons. This is similar to the low loss electron phenomenon, which is readily observed in PIA experiments in the laboratory.
In ball lightning, electron recombination times in air, hastened by electron attachment to oxygen and water, are on the order of 10 microseconds. But appreciable levels of ionization appear to precede the main lightning discharge by 10 msec and persist for periods of 10 msec or longer afterwards. This is called the stepped leader phenomenon. This phenomenon and the unexplained interval between discharges is commonly observed in lightning storms.
Several theoretical models have been proposed in the past for ball lightning. These models suggest the involvement of RF radiation. An early theory explained ball lightning as an evacuated microwave resonant cavity surrounded by a layer of plasma. Another theory proposed that vorticity can play a part. A recent theory describes ball lightning as an electromagnetic knot, with tangled magnetic fluxes. The electromagnetic knot model predicted an expansion of the plasma as it cools, in the limit of infinite conductivity.
The process of plasma formation in air by microwaves has also been extensively investigated, both experimentally and theoretically. As a result, it is known that the formation of plasmas in air, O2, and N2 are fairly similar. Breakdown is achieved at lower field strengths with lower frequencies: approximately 1000 V/cm will achieve breakdown in room air at 0.992 GHz, whereas approximately 3000 V/cm is required at 9.4 GHz.
A number of researchers have produced PIA plasmas using high-frequency electromagnetic fields at atmospheric pressure to simulate ball lightning. Kapitza originally formulated a theory that ball lightning forms from RF waves in the atmosphere. Tesla made the earliest report of an artificial creation of ball lightning. Later, Powell and Finkelstein succeeded in making spherical discharges that would separate from the electrodes where they formed. They used 75 MHz RF at 20 kW and a 15-cm-diameter Pyrex tube to form the plasmas. Powell and Finkelstein found that the large volume plasmas produced in those experiments persisted for as much as 0.5 seconds after termination of the ionizing radiation.
In more recent experiments, researchers used a 1-5 kW 2.45-GHz power source to drive a resonant cavity, but did not restrict the physical extent of the plasmas formed. The researchers created large air discharges in the resonant cavity. These discharges were often augmented by ordinary combustion. Other researchers have used helium gas as a plasma medium at atmospheric pressure.
In previous experiments for creating PIA plasmas, high-power sources, resonant cavities, or specialized gases were needed in order to create large plasmas at atmospheric pressure. No method or device currently exists for creating PIA plasmas with commercially available equipment, such as commonly available gases and power sources. Further, previous research efforts have not succeeded in measuring the properties of the created plasmas. Accordingly, there currently exists a need for apparatus and methods for creating PIA plasmas efficiently and economically, and for measuring the properties of the created PIA plasmas.